Regensburg 2016 – scientific programme
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O: Fachverband Oberflächenphysik
O 86: Ultrafast Surface Dynamics I
O 86.11: Talk
Thursday, March 10, 2016, 17:30–17:45, S052
Tracing thermal and nonthermal phase transition by pump-probe schemes. Part I: material behavior under irradiation — •Nikita Medvedev1, Victor Tkachenko1, and Beata Ziaja1,2 — 1CFEL at DESY, Notkestr. 85, Hamburg, Germany — 2Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Krakow, Poland
We study theoretically effects of thermal and nonthermal phase transitions in semiconductors irradiated with femtosecond laser pulses. Our developed hybrid code XTANT combines: (i) Monte Carlo method tracing highly-excited nonequilibrium electrons; (ii) Boltzmann collision integrals to calculate energy exchange between atoms and low-energy electrons; (iii) tight binding molecular dynamics following atomic motion on the evolving potential energy surface. This combined approach allows to model both, thermal (triggered by heating of atoms via electron-phonon coupling) and nonthermal (triggered by changes of the interatomic potential through electron excitation) phase transition.
In the first part of the report, we present the XTANT model and discuss the behavior of irradiated semiconductors occurring at different timescales. We demonstrate how the electronic kinetics at femtosecond timescales impacts the atomic dynamics. The nonthermal phase transitions occur within a few hundred femtoseconds. Typically at lower excitation doses, thermal heating may lead to structural changes at picoseconds timescales. Examples of irradiated silicon and GaAs will be presented and compared to experiments.